This application claims priority of German application number 101 60 955.8 filed Dec. 12, 2001.
1. Field of the Invention
The invention relates to a wheel balancing method and apparatus, and more particularly to a method for positioning a rotor, in particular a motor vehicle wheel, which is rotatably mounted on an wheel balancing machine and can be driven by means of an electric motor, as a desired position for unbalance compensation.
2. Related Art
In such a method which is known from DE 196 36 268 C2, the rotor is driven again after the unbalance compensation has been carried out in a first compensation plane, and after the drive has been switched off said rotor is braked in such a way that the resulting braking distance corresponds to the remaining difference angle up to the second unbalance compensation position. The braking of the rotor can be brought about by reversing the torque of the electric motor which is used to drive the main shaft of the unbalance machine.
DE 100 00 235 A1 discloses an unbalance machine whose main shaft is connected to a single-phase alternating current motor, two stator windings being provided which are supplied with current with a phase difference of 90xc2x0. The phase difference of the power supply is brought about by means of at least one capacitor which can be connected in series with one of the two stator windings depending on the direction of rotation.
The object of the invention is to provide a wheel balancing method and apparatus with which a rotor can easily be moved from a stopped position, in particular, an unbalance compensation position, as which unbalance compensation has already been carried out, into second position at which a subsequent unbalance compensation can be carried out.
According to the invention, this object is achieved by a method in which the rotor is driven to the second unbalance compensation position, with the electric motor switched off, into a freely running rotation by an externally acting force which is, for example, applied manually, in particular by an operator. During this freely running rotation, the electric motor is switched on in order to generate a counter torque which acts counter to the direction of rotation of the rotor. The switching on takes place in such a way that the braking distance which is brought about by the counter torque corresponds to the angle of rotation which is traveled over to the unbalance compensation position. The electric motor is switched off in the compensation position.
The angle of rotation at which the counter torque is switched on is determined from the braking effect which was detected when the rotor was braked into the previous stop position, in particular the unbalance compensation position, and the instantaneous rotational speed (respective actual rotational speed) is determined during the freely running out rotation of the rotor.
While the main shaft of the balancing machine is driven in order to drive the rotor, the alternating current motor is to be adjusted to the measuring rotational speed which is suitable for the unbalance measurement, supplied with current under the control of an alternating voltage controller. The stator windings of the alternating current motor are supplied with alternating current here by the acceleration to the measuring rotational speed and during the braking of the motor at which a torque is generated. The alternating voltage controller, in which both the longitudinal switch and the commutation switch are formed from two semiconductor switches which are connected in an anti-serial configuration, is provided for this purpose. Suitable semiconductor switches are power switches, in particular IGBTs or MOSFETs. The alternating current motor is preferably embodied as a single-phase alternating current motor which has two stator windings which are supplied with current with a phase difference of, in particular, 90xc2x0.
The semiconductor switches of the alternating voltage controller are switched on and off under the control of a pulse width modulation (PWM). By means of the pulse duty factor and the clock frequency of the pulse width modulation it is possible not only to control the acceleration of the rotor to the measuring rotational speed but also to set the braking effect, when the torque is generated, in such a way that the rotor is stopped, in particular in the respective compensation position. This also applies to the case in which, after the compensation process in a first compensation position, the rotor is selectively moved into a second compensation position, the rotor being made to rotate in a freely running out fashion by means of a force which is applied from the outside by an operator, in particular manually, and the controlled braking operation is switched on.
As a result of the braking using a counter torque which is generated by alternating current, a significantly higher braking effect is obtained than when direct current is supplied to the motor when the rotor is braked.
For this reason, with the invention the effect of a position brake with which the rotor is stopped in the respectively desired position, in particular compensation angle of rotation position, is obtained without additional installations.